Laser radiation for semiconductor photolithography is typically supplied as a series of pulses at a specified repetition rate. In order to achieve process uniformity, it is desirable that the laser be able to meet a set of performance specifications such as a bandwidth, wavelength, and energy stability under all anticipated operating conditions. These laser performance parameters may be affected by the repetition rate at which the laser performs. Because of this it cannot be assumed that the laser will meet performance specifications at all of the repetition rates at which it is able to operate. It may be desired, however, to have the option of being able to operate at different repetition rates. For example, a common method of changing the output power of the laser is to reduce the repetition rate rather than to reduce the output energy per pulse.
It is possible to engineer around unknown variation of performance with repetition rate by operating the laser at a fixed repetition rate (for example, 6 kHz). This has the disadvantage, however, that if it is desired to use a lower effective repetition rate, the scanner associated with the laser source must block or otherwise attenuate pulses, because the actual number of pulses originating from the laser will remain the same. This means that more laser pulses are used for wafer production than would be necessary if the actual laser repetition rate could be reduced when desired.
There is thus a need be able to provide a laser source that can be operated at any one of a multiplicity of available repetition rates without concerns that the laser source may not be operating within acceptable performance specifications.